The present invention relates to a method for producing a pleatable filter material from a nonwoven fabric, having spacers for pleated folds made of the filter material itself.
A method for producing a pleated filter medium with stamped protrusions is described, for example, in U.S. Pat. No. 3,531,920. According to this method, the filter material is passed from a roll to a press which includes two heated cylinders rotating in opposite directions. The cylinders are provided with meshing protrusions and the corresponding recesses, and the filter material passed through between them is durably shaped by deep-drawing. The shaping process influences the structure of the filter material in the deep-drawn area, and thereby changes the original filtering properties in the areas important to the filtering.
An improvement in the method described above is achieved by the method described in European Published Patent Application No. 0 429 805. In this method, a flat filter medium is gathered transversely to the running direction by rolls, and subsequently, elongated protrusions are stamped into the gathered material by the dies of a shaping device. The gathering prevents the additional material, required by the stamped protrusions, from leading to tensions in the material and the structure from being changed in the deep-drawn area of the filter medium. But this method also has the disadvantage that the spacers formed by stamping can only be impressed over a part of the filter surface.
German Published Patent Application No. 196 30 522 describes scoring and bonding a formed fabric made of stretched and unstretched synthetic fibers between profiled calender rolls. By this method a filter material could be produced from a nonwoven fabric without having a change in the homogeneity of the nonwoven fabric appear. The desired scoring is, however, not fully satisfactory, because outside the calender rolls it is partially leveled again by the tractive force toward the material transport.
Therefore, it is one object of the present invention to provide a method for producing a filter material in which the finished filter material has great stability. The formed spacers should keep their shape and show great stability, both during the production process and in later filtering operation, under the influence of mechanical and/or thermal stress.
The above and other beneficial objects of the present invention are attained by providing a method in which a formed fabric made of stretched synthetic fibers and a thermoplastic and/or thermally cross-linked binding agent is heated in an oven to a temperature that is at least in the softening temperature range and/or the cross-linking temperature range of the binding agent, and in which, subsequently, the formed fabric is formed between profiled calender rolls and cooled simultaneously. This method yields a filter material with the utmost stability and shape-retaining ability, the filtration properties being maintained simultaneously to the highest degree.
The stretched synthetic fibers are required in order to provide the filter material generally with the desired firmness. The binding agent is applied to attain a stable structure of the fibers among one another. This may require the use of a thermoplastic as well as a thermally cross-linking binding agent. The temperature applied to the formed fabric in the oven is set so that, in a formed fabric having a thermoplastic binding agent, the softening temperature range of the binding agent is at least reached. Consequently, the single fibers are connected to one another by the binding agent.
In the case of a thermally cross-linking binding agent, the oven temperature is raised until the cross-linking temperature range is reached, in which controlled cross-linking occurs. The cross-linking is taken to the point at which a stable structure of the formed fabric is present. The formed fabric thus treated is then delivered to the calender rolls, which have a lower temperature than the oven temperature. During calendering, the formed fabric is cooled and simultaneously formed, that is, the spacers are put in. After leaving the calender rolls, the formed fabric has a very high remaining stability.
The method of producing the formed fabric and putting in a thermoplastic binding agent may include the step of adding bicomponent fibers having a thermoplastic fiber component into the nonwoven fabric during its production. For this purpose, suitable bicomponent fibers are fibers having a core structure, a sheath structure, an island structure or a side-by-side structure. The bicomponent fibers may be introduced into the formed fabric by mixing the fiber components or by including them in the formed fabric. By heating in the oven, the thermoplastic component of the bicomponent fibers is softened or melted, and the desired connection of the fibers in the formed fabric occurs.
The temperature may be selected so that at least softening or fusing of the binding component occurs. This is normally reached in the range between 90xc2x0 C. and 240xc2x0 C. The desired connection of the fibers is caused by the melting and resolidification of the polymer while cooling in the calender.
It is also possible to use thermoplastic binding agents or cross-linking binding agents in another, e.g., powder, dispersion, solution, which are introduced into the formed fabric or applied to the formed fabric. The latter, for example, may be accomplished by spraying the binding agent onto the formed fabric. Alternatively, the binding agent may be introduced into the formed fabric by steeping or impregnating.
The oven temperature is set so that the connecting process occurs reliably in as short a time as possible. The temperature is set to the softening temperature range or to the melting point range according to the type of thermoplastic binding agent. Generally, the oven temperature is set to between 100xc2x0 C. and 240xc2x0 C. depending on the binding agent to be used. With a core/sheath structure of the bicomponent fibers, a temperature is selected which is below the melting point of the core, but a reading of 25xc2x0 C. below the melting point of the core component should not be exceeded. The sheath is heated to its own melting temperature, and this results in a good bonding of the sheath layer.
The temperature of the calender rolls is selected to be below the softening temperature range of the binding agent. In general, the temperature range is approximately 70xc2x0 C. to 150xc2x0 C. In order to make the procedure as simple as possible, a temperature range of 80xc2x0 C. to 90xc2x0 C. may be used. The temperature of the roller has to cool the formed fabric so that durable forming can be achieved. The temperature of the calender rolls and the residence time of the formed fabric between the calender rolls is to be set corresponding to the binding agent.
In addition to the shaping of the unbonded formed fabric into a three-dimensional structure, a calibration of the formed fabric as to uniform thickness in the calender occurs at the same time. Even though the formed fabric is already losing volume under the effect of heat in the oven, the final calibration occurs only between the calender rolls.
One example embodiment of the present invention provides that the preformed fabric is pressed together uniformly by the calender rolls without inhomogeneities occurring on its surface. The desired spacers may be of various specific embodiments. The example embodiment provides that the spacers are formed as a sine-shaped wave in the filter material. The wave crests and troughs are arranged in the direction of travel.
Profiled calender rolls are used for this, as described, for example, in German Published Patent Application No. 196 30 522. The scoring is uniform over the entire cross-section, and the filter material also has a uniform thickness. However, by using an equivalent formation of the calender rolls, it is also possible to submit the formed fabric to an increased compression at predefined areas of the cross-section, so as to achieve a greater stiffness in the filter material at these locations.
Formation of the spacers is also possible by elevations and/or indentations set apart from one another. Particular filter materials may be made in this manner, and various geometries are possible.
The scoring or elevations in the filter material may be selected in accordance with the ultimate use of the filter material. In general, their height should correspond to at least a quarter of the thickness of the filter material.